1. Field of the Invention
This invention relates generally to hybrid nuclear medical imaging, wherein nuclear medical images are co-registered or fused with images of the same region obtained with a different imaging modality, such as Computerized Tomography (CT), Magnetic Resonance Imaging (MRI) and Ultrasound (US). More specifically, this invention relates to improvement in image registration accuracy between nuclear medical images and images obtained with a different imaging modality such as CT, where both modalities are obtained with a single hybrid imaging apparatus.
2. Background and Prior Art
In nuclear imaging, a patient is injected with or swallows a radioactive isotope which has an affinity for a particular organ, structure or tissue of the body. Gamma rays are then emitted from the body part of interest, are collimated by a collimator so that only gamma photons traveling in a direction perpendicular to the surface of a detector head are allowed to impinge on the detector head, and are detected by a gamma camera apparatus including the detector head, which forms an image of the organ based on the detected concentration and distribution of the radioactive isotope within the body part of interest. Nuclear images may be obtained using single photon emission (either planar or Single Photon Emission Computed Tomography (SPECT)) and Position Emission Tomography (PET). Planar imaging essentially compresses a three-dimensional radiation field onto a two-dimensional image plane, while SPECT and PET produce multiple image “slices,” each representing a different plane in a three-dimensional region, such that when the slices are considered collectively, a three-dimensional image of the region may be studied.
Nuclear imaging is particularly suited to studying function and activity of tissue and organs, while other imaging modalities such as CT and MRI are more oriented to providing anatomical and structural information. Consequently, it is particularly useful in certain studies such oncological and cardiology studies to use SPECT or PET imaging for diagnostic purposes, and to align or register the nuclear image with a medical image from another modality such as CT or MRI, which offers better anatomical information. Such a fused image, for example, enables the clinician to determine the anatomical position of a lesion displayed by the nuclear image more accurately, and the organs and structures affected to be ascertained with higher accuracy and confidence.
In recent years there has been considerable interest in development of techniques to co-register or align medical images of different modalities, such as PET and CT images, to thereby combine both functional and anatomical features in a single image. See, eg., U.S. Pat. No. 6,490,476 to Townsend et al. In particular, techniques such as landmark registration or external marker registration are generally known in the art. Such techniques require either a significant amount of human interpretation of two separate images or require the use of external markers attached to the patient while two different imaging procedures are performed.
Recently, Siemens Medical Solutions introduced a new concept in hybrid imaging technology known as TruePoint™ SPECT-CT, which combines the functional sensitivity of SPECT with the anatomical detail of diagnostic multi-slice CT in a single imaging apparatus, thereby providing clinicians with imaging clarity as well as diagnostic confidence. Such hybrid imaging technology allows the capture of comprehensive, accurate diagnostic information on both the molecular and anatomical levels, within a single scan procedure.
In addition to the dual modality capability that enables SPECT attenuation correction and SPECT/CT image overlay, another significant advantage of the hybrid system platform (known as Symbia™) is a versatile mechanical system that allows the detectors to scan at optimal positions for various studies, such as cardiology studies.
The challenges involved in SPECT/CT image registration have among their causes the difference in resolution between CT images and SPECT images. In particular, the resolution of a CT image and its geometrical accuracy are high, while the resolution of a SPECT image is comparatively low, and the geometrical distortion of the SPECT image has historically received less attention. This mismatch in resolution between the two modalities is one cause of inaccuracy in image registration of the images produced by the different modalities.
Among the various factors that cause SPECT image distortion, detector head deflection and alignment issues are among the largest sources of error. The more versatile mechanical system of the Symbia™ platform amplifies the potential for errors because it introduces the capability for new detector positions and accordingly the conventional projection geometry assumptions used in image reconstruction are no longer applicable, leading to potential for image blurring and distortion. Examples of such head deflection and misalignment errors are shown in FIGS. 8(a)-8(c) and FIGS. 9(a)-9(b).
While one way to reduce such problems is mechanical measurement of the detector heads, such a procedure is costly and still cannot eliminate the errors caused by the electronics and other system factors.
Accordingly, there exists a need in the art for improvement in correction of projection image positional errors in order to improve the accuracy and quality of such images when used in SPECTICT image overlays.